1. Field of the Invention
This invention relates in general to earth boring drill bits and in particular to improved bearings and means for retaining the rotatable cutters on such bits.
2. Prior Art
The success of rotary drilling enabled the discovery of deep oil and gas reservoirs. The rotary rock bit was an important invention which made that success possible. Only the soft formations could be commercially pentrated with the earlier drag bit, but the two cone rock bit invented by H. R. Hughes, U.S. Pat. No. 939,759, drilled the hard cap rock at the Spindletop Field near Beaumont, Tex. with relative ease.
That distant invention, within the first decade of this century, could drill a scant fraction of the depth and speed of the modern rotary rock bit. If the original Hughes bit drilled for hours, the modern bit drills for days. Bits today sometime drill for miles instead of feet. Many improvements contributed to the impressive improvement in the performance of rock bits.
The original bit of Hughes had rotatable cutters, frictional bearing surfaces and ring-type retainers generally threaded for retention in the cutter. Because of the difficulty in providing seals with long life, relatively large lubricant reservoirs were required. By the decade of the thirties, drill bits with anti-friction bearing that were unsealed became commercially successful and replaced the sealed and lubricated drill bits. This type drill bit may be seen in the U.S. patent to Lewis E. Garfield et al, U.S. Pat. No. 2,030,442.
G. O. Atkinson et al obtained by U.S. Pat. No. 3,075,781 on a seal that mimimized leakage and required only a small reservoir of lubricant. The pressure compensator is an improvement that minimizes the pressure differential across the seal and increases its reliability. A recent version of the pressure compensator and pressure relief system is shown in the U.S. Patent of Stuart C. Millsapps, Jr., U.S. Pat. No. 3,942,596. E. M. Galle patented an O-ring type seal, U.S. Pat. No. 3,397,928, that ultimately made friction bearings once again feasible in drill bits. An illustration of a recent bearing configuration is shown in the U.S. Pat. No. 3,922,038 issued to S. R. Scales. This bit has a cylindrical, friction bearing and a cutter retained by a ball bearing similar to that shown in the above patent to Lewis E. Garfield et al.
A ball bearing in a lubricated rock bit bearing has inherent disadvantages. A failure in any one of the numerous balls may permit metallic fragments to enter the friction bearing, with near certain damaging results. Metallic particles will often damage the seal ring, causing lubricant loss and rapid bearing failure. The necessity for a hole drilled into the ball race for introduction of the balls, retained by a welded plug, adds complexity to the bit that provides additional room for manufacturing error. The rock bit is the focal point of an expensive drilling process that has a low tolerance for failure.
There is shown in the prior art rock bit bearing and retainer means that are totally frictional; that is, without anti-friction ball or roller bearings. The original Hughes bit had friction bearings exclusively. A few of the many bits with only frictional bearings will be mentioned briefly.
F. L. Scott in U.S. Pat. No. 1,803,679 discloses a lubricated, tapered frictional rock bit bearing that retains its cutter by means of a resilient snap ring. J. C. Wright et al in U.S. Pat. No. 2,049,581 discloses a cutter rotatably secured with a snap ring to a bearing shaft releasably connected to the legs or head sections of a bit. The U.S. Pat. No. 2,814,465 to W. G. Green discloses a lubricated frictional bearing having tapered and cylindrical portions joined by suitable means and secured to a cutter by a retainer ring. A lock ring is shown in Edward B. Williams, Jr.'s U.S. Pat. No. 3,844,363 for retaining a cutter and shaft with frictional bearings to a rock bit. A frictional bearing with tapered and cylindrical portions is disclosed by E. M. Galle in his U.S. Pat. No. 3,361,494, along with frictional plug lock and pin lock retaining systems. A recent revival of the earlier seen threaded ring retainer is seen in U.S. Pat. No. 3,971,600 of Murdoch et al. Notwithstanding this array of ideas, the only significantly successful rock bit in commerce today uses a ball bearing retainer.
The threaded retainer ring has some disadvantage that may have limited its success. The threads act as stress raisers and the ring must be secured against rotation while the cone is mated with it. This generally requires a drilled hole in the head section.
Snap rings may have stress raising groove configurations and exhibit a tendency to retract during drilling into the assembly groove and permit accidental cutter loss. When a cutter is thrust inward, as when reaming for example, the loading of the ring causes stresses that tend to urge the ring into the assembly groove, with the possibility of cutter loss.